Devices for converting radiant energy, such as optical energy, into electric energy presently generally take two forms, viz.: (1) semiconductors relying upon a barrier layer mechanism, and (2) pyroelectric devices wherein a ferroelectric is cyclically heated and cooled to provide corresponding changes in the capacitance and resistance of a capacitor including the ferroelectric.
Typically, the barrier layer semiconductor devices have relatively heavy doped semiconductor layers with energy gaps that, in essence, absorb certain wavelengths of interest and convert the energy of the absorbed wavelengths into electric energy. These devices are utilized as radiant, optical energy detectors for specific wavelengths of interest, as well as power generating solar cells. The major disadvantage of the semiconductor devices as radiant energy detectors is that the semiconductor element must usually be maintained at cryogenic temperatures to function effectively. It is frequently difficult to maintain a semiconductor device at a cryogenic temperature, whereby the usefulness of semiconductor radiant energy detectors is often limited. The major disadvantage of the semiconductor devices as solar energy converters is that such devices are relatively inefficient in converting the solar energy into electrical energy. The typical, actual maximum efficiency of such converters is generally on the order of 10%.
Pyroelectric devices are generally characterized by a ferroelectric dielectric that is positioned between a pair of electrodes to form a capacitor responsive to the optical energy. Typically, the ferroelectric material is periodically heated and cooled to cause a periodic variation in the capacitance and resistance of the capacitor. Since the ferroelectric materials have dipole layers extending completely through the dielectric, i.e., from one electrode to the other electrode, the dielectrics are strongly piezoelectric, making them sensitive to vibrations. Thereby, the pyroelectric detectors have a tendency to be noisy and frequently have relatively low signal to noise ratio outputs. In addition, the pyroelectric detectors often have detectivities below the level of the radiation impinging on the dielectric, thereby limiting their application in many systems.
In my copending application entitled "Apparatus For Converting Radiant Energy Into Electric Energy", Ser. No. 631,689, filed Nov. 13, 1975, there is disclosed an improved radiant energy detector and solar energy converter having higher detectivity and sensitivity than prior art detectors and greater efficiency than prior art energy converters. In the device disclosed in my copending application, a capacitor includes an ionic dielectric having a dipole layer only on or near the dielectric surface. The dielectric is generally selected from the group consisting of the rare earth trifluorides and trichlorides, and is preferably lanthanum trifluoride. A possible problem with the use of this class of materials is that it has relatively low breakdown voltages, on the order of 5 to 10 volts, regardless of the thickness of the dielectric layer; the breakdown voltage is dependent exclusively upon the dielectric material. A further possible disadvantage of the device disclosed in the copending application may be the difficulty of obtaining the specified rare earth trifluorides and trichlorides.